Methods and circuitry for correcting temperature-induced errors in microbolometer focal plane array

ABSTRACT

Correction for temperature-induced non-uniformities in the response characteristics of the microbolometers in an infrared focal plane array (FPA) is performed by applying a non-uniform corrective bias to the individual microbolometers. The corrective bias is applied either before or during the bias or integration period during which the detectors are sampled. The bias-correction can be applied to two-dimensional detector multiplexers at each column amplifier input, the reference potential for each column amplifier or the voltage supply for each detector element. The magnitude of each corrective bias is determined by calibrating the detectors at different temperatures and different levels of incident infrared radiation. According to another aspect of this invention, a microbolometer which is thermally-shorted to the substrate on which the read out integrated circuit (ROIC) is formed is used along with the sensing microbolometer to compensate for variations in temperature. In some embodiments, an adjustable voltage is applied to the thermally-shorted microbolometer to provide an offset correction. Circuitry for providing on-ROIC substrate temperature control is also described. This invention allows the operation of a microbolometer FPA over a wider range of device substrate temperatures and thereby significantly reduces the complexity and cost of the system as compared with the conventional technique of cooling the FPA.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.08/799,663, filed Feb. 11, 1997, now U.S. Pat. No. 5,756,999, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

During the 1880's, an infrared detector called the bolometer wasdeveloped. The bolometer operates on the principle that the electricalresistance of the bolometer material changes with respect to thebolometer temperature, which in turn changes in response to the quantityof absorbed incident infrared radiation. These characteristics can beexploited to measure incident infrared radiation on the bolometer bysensing the resulting change in its resistance. When used as an infrareddetector, the bolometer is generally thermally isolated from itssupporting substrate or surroundings to allow the absorbed incidentinfrared radiation to generate a temperature change in the bolometermaterial.

Modern microbolometer structures were developed by the HoneywellCorporation. For a recent summary of references see U.S. Pat. No.5,420,419. Microbolometer arrays are typically fabricated on monolithicsilicon substrates or integrated circuits by constructingtwo-dimensional arrays of closely spaced air bridge structures coatedwith a temperature sensitive resistive material, such as vanadium oxide,that absorbs infrared radiation. The air bridge structure provides goodthermal isolation between the microbolometer detector and the siliconsubstrate. A typical microbolometer structure measures approximately 50microns by 50 microns.

Microbolometer arrays can be used to sense a focal plane of incidentradiation (typically infrared) by absorbing the radiation and producinga corresponding change in the temperatures and therefore resistances ofeach microbolometer in the array. With each microbolometer functioningas a pixel, a two-dimensional image or picture representation of theincident infrared radiation can be generated by translating the changesin resistance of each microbolometer into a time-multiplexed electricalsignal that can be displayed on a monitor or stored in a computer. Thecircuitry used to perform this translation is commonly known as the ReadOut Integrated Circuit (ROIC), and is fabricated as an integratedcircuit in the silicon substrate. The microbolometer array is thenfabricated on top of the ROIC. The combination of the ROIC andmicrobolometer array is commonly known as a microbolometer infraredFocal Plane Array (FPA). Microbolometer FPAs containing as many as320×240 detectors have been demonstrated.

Individual microbolometer detectors will have non-uniform responses touniform incident infrared radiation, even when the bolometers aremanufactured as part of a microbolometer FPA. This is due to smallvariations in the detectors' electrical and thermal properties as aresult of the manufacturing process. These non-uniformities in themicrobolometer response characteristics must be corrected to produce anelectrical signal with adequate signal-to-noise ratio for image displayor processing.

Under the conditions where uniform electrical bias and incident infraredradiation are applied to an array of microbolometer detectors,differences in detector response will occur. This is commonly referredto as spatial non-uniformity, and is due to the variations in a numberof critical performance characteristics of the microbolometer detectors.This is a natural result of the microbolometer fabrication process. Thecharacteristics contributing to spatial non-uniformity include theinfrared radiation absorption coefficient, resistance, temperaturecoefficient of resistance (TCR), heat capacity, and thermal conductivityof the individual detectors.

The magnitude of the response non-uniformity can be substantially largerthan the magnitude of the actual response due to the incident infraredradiation. The resulting ROIC output signal is difficult to process, asit requires system interface electronics having a very high dynamicrange. In order to achieve an output signal dominated by the level ofincident infrared radiation, processing to correct for detectornon-uniformity is required.

Previous methods for implementing an ROIC for microbolometer arrays haveused an architecture wherein the resistance of each microbolometer issensed by applying a uniform voltage and current and a resistive load tothe microbolometer element. The current resulting from the appliedvoltage is integrated over time by an amplifier to produce an outputvoltage level proportional to the value of the integrated current. Theoutput voltage is then multiplexed to the signal acquisition system.

Gain and offset corrections are applied to the output signal to correctfor the errors which arise from the microbolometer propertynon-uniformities. This process is commonly referred to as two-pointcorrection. In this technique two correction coefficients are applied tothe sampled signal of each element. The gain correction is implementedby multiplying the output voltage by a unique gain coefficient. Theoffset correction is implemented by an adding a unique offsetcoefficient to the output voltage. Both analog and digital techniqueshave been utilized to perform two-point non-uniformity correction.

The current state-of-the-art in microbolometer array ROICs suffers fromtwo principal problems. The first problem is that the large magnitude ofthe microbolometer introduced non-uniformities in the ROIC output signalrequires a large instantaneous dynamic range in the sensor interfaceelectronics, increasing the cost and complexity for the system. Currentadvanced ROIC architectures incorporate part of the correction on theROIC to minimize the instantaneous dynamic range requirements at theacquisition systems interface.

The second problem is that the application of a fixed coefficienttwo-point gain and offset correction method to minimize arraynon-uniformity works well only for a very small range of substratetemperatures, on the order of 0.005 to 0.025 degrees Kelvin. In order tomaintain the substrate temperature within this range, a thermo-electriccooler, temperature sensor, and temperature control electronics arerequired, again adding to system cost and complexity.

Microbolometer Operation

FIGS. 1A, 1B and 1C illustrate three possible configurations formicrobolometer detectors. Incident infrared radiation 1 is projectedonto each of the microbolometer detectors. The microbolometer detector2, shown in FIG. 1A, is thermally-shorted to the substrate material.This is a common form of bolometer and is representative of mostsingle-detector bolometers and thermistors. Microbolometers can bemanufactured to provide a high thermal conductivity to the substrate, orthis property can be introduced through post-processing whereby athermally conductive material is selectively applied to thesemicrobolometer detectors. Although this detector is thermally connectedto the substrate, the resistance properties and temperature coefficientof resistance (TCR) of these detectors are equivalent to the thermallyisolated microbolometer detector 3, shown in FIG. 1B. These detectorshave a high TCR (1% to 5%/° C.) which is designated by the arrow in theresistor symbol.

The thermally isolated microbolometer 3 is most commonly used to senseinfrared radiation. Microbolometer 3 is thermally isolated from itssupporting substrate or surroundings to allow the absorbed incidentinfrared radiation to generate a temperature change in themicrobolometer material. In FIG. 1B, this isolation is designated by thedashed square box around the detector.

The final configuration, shown in FIG. 1C, is the shieldedmicrobolometer 4. The shielded microbolometer 4 is identical to theisolated microbolometer 3 with the exception that incident infraredradiation 1 is shielded from the microbolometer. The radiation shield isdesignated by a solid line 5 in FIG. 1C.

The principles of operation for the microbolometers shown in FIGS. 1A-1Care as follows. The temperature of the non-isolated microbolometer 2 isdominated by the high thermal conductivity to the substrate. Thereforeincident infrared radiation and electrical power dissipated in themicrobolometer have little effect on the temperature of themicrobolometer. Microbolometer 2 has the same high TCR as the thermallyisolated microbolometer 3 and therefore provides a high sensitivityreference to the substrate temperature.

The thermally isolated microbolometer 3 changes temperature in responseto the incident radiation level, changes in the substrate temperature,and the electrical power dissipated in the detector during themeasurement of the microbolometer resistance. The heating due toresistive measurement is referred to as self-heating. As fabricated, thethermally isolated microbolometer is not perfectly insulated from thesubstrate. Therefore the temperature of the thermally isolatedmicrobolometer does track the substrate temperature to some extent,although the rate of temperature change due to this mechanism is muchslower than those due to incident radiation or self-heating.

The shielded isolated microbolometer 4 does not change temperature inresponse to the incident radiation level, but does change temperature asa result of self heating and temperature changes in the substrate.

FIGS. 2-4 illustrate three commonly used techniques for measuring theresistance of the microbolometer detector. FIG. 2 shows the appliedvoltage method of sensing the detector resistance. An applied voltage isused to generate a current in the circuit designated Iout. By measuringthe current Iout the resistance of the microbolometer detector can bedetermined. The relationship between the applied voltage and measuredcurrent is defined by Ohms Law. ##EQU1## Where Iout is the measuredcurrent, Vapplied is the applied voltage and Rbolometer is themicrobolometer detector resistance.

A second method for measuring the microbolometer resistance is shown inFIG. 3. Here a constant current is applied to the microbolometerdetector 3, and the voltage that develops across the microbolometer as aresult is measured. Again Ohms Law defines the relationship between theapplied current and the measured voltage.

    Vout=Iapplied*Rbolometer

A third method for measuring the microbolometer resistance is shown inFIG. 4. This circuit includes a resistive load 6. A voltage is appliedacross the series combination of the microbolometer 3 and the load 6.The microbolometer resistance can be determined by measuring the voltageacross the microbolometer. The following expression describes themicrobolometer resistance as a function of applied voltage, loadresistance, and the measured voltage across the microbolometer. ##EQU2##where Rload is the value of load resistor 6.

These circuit implementations can be used to measure infrared radiationincident on the microbolometer by sensing the change in microbolometertemperature due to the optical (infrared) energy absorbed by thedetector. The temperature rise in the microbolometer detector due toself-heating generally is significantly larger than the temperature riseresulting from the incident infrared radiation. The relatively smallcontribution of incident radiation to the change in microbolometerresistance is difficult to detect. For this reason, it is desirable toincorporate more complex circuits using in-circuit reference schemes inorder to minimize the contribution of self-heating to the output signal.In the case of the resistive load circuit approach (FIG. 4), the loadresistor 6 may be designed to have a low temperature coefficient ofresistance, or it may be thermally-shorted to the substrate, or shieldedfrom incident infrared radiation.

Circuit bridge concepts have been designed to minimize the errors inresistance measurement due to self-heating. FIG. 5 illustrates amicrobolometer bridge concept used to isolate and measure only the levelof incident infrared radiation. Here a microbolometer that is thermallyisolated and shielded from incident radiation is shown in a bridgeconfiguration with three conventional low TCR resistors, 6a, 6b and 6c.The output voltage in FIG. 5 will remain relatively constant in spite ofa change in ambient temperature, because the increase in resistance ofthe microbolometer detector and the shielded microbolometer willincrease by approximately the same amount and keep the voltage dropsacross the elements of the bridge circuit approximately unchanged.

Microbolometer Focal Plane Arrays

In systems where a single detector is employed, two conductive leads canbe attached to the microbolometer material providing a means ofconducting current through the microbolometer to sense its resistance.FIG. 6 illustrates the electrical connection to the microbolometerdetector. In this case, a thermally isolated microbolometer 3 is shownin the presence of incident infrared radiation 1 with two leadsconnecting to microbolometer terminals R+ and R-. FIG. 7 shows thephysical implementation of a microbolometer of the kind developed byHoneywell. The R+ and R- electrical connections to the microbolometerare created at the ends of the legs 9 where the microbolometer comes incontact with the substrate 8.

In cases where it is desired to sense the resistance or temperature ofan array of microbolometer detectors it becomes physically impracticalto provide individual wire lead connections for each detector. FIGS. 8and 9 illustrate the method of interconnecting to a microbolometerdetector array. Shown in FIGS. 8 and 9 is a three-by-three detectorarray requiring nine positive and negative interconnects. Interconnectsfor the individual microbolometer detectors 3 in the array are createdas part of the fabrication process, and contact the circuitry in thesilicon substrate 8.

Large two-dimensional arrays of microbolometers can utilize a Read OutIntegrated Circuit (ROIC) to provide the required bolometer interface.The ROIC incorporates circuitry that is placed in spatial proximity tothe detectors to perform the functions of the detector interface andmultiplexing. The circuitry associated with a particular microbolometerdetector is often located in the silicon substrate directly beneath thedetector and is referred to as the unit cell.

By time-multiplexing signals of the microbolometer detectors the numberof required electrical interconnect leads can be greatly reduced. FIG.10 shows schematically a one-dimensional multiplexing scheme for amicrobolometer array. Three microbolometer detectors 3 are multiplexedto a single column amplifier 15. A row enable line 16 is used to controlan address switch 10 in a unit cell 14. This allows selective connectionof the unit cell bolometers to the column amplifier 12. The currentthrough each microbolometer detector will be sequentially sampled forintegration by the amplifier. The order of sequencing and the timeperiod of each sample is determined by the sequencing and duration ofeach row enable signal's active period. In this embodiment a uniformbias 11 is applied to the microbolometer array at the time each detectoris addressed. A conventional inverting amplifier 12, and its feedbackelement 13 is shown in the column amplifier block 15. In an actual ROICthe address switches 10 shown in FIG. 10 would be implemented as MOS orbipolar transistors, and are shown as switches to simplify theillustration.

Multiplexing can be expanded to a second dimension by arraying theone-dimensional configuration shown in FIG. 10. The resultingtwo-dimensional three-by-three configuration is shown in FIG. 11. Thetwo-dimensional array is implemented by adding column interconnects andincorporating column multiplexing switches 18. Column amplifiers 15shown in FIG. 10 have been modified to incorporate a sample and holdstage 12A to allow time-simultaneous sampling of the signal in thecolumn dimension. Outputs from sample and hold stages 12A are selectedby the column enable signal 19 that controls the column switch 18. Anoutput line 17 common to all columns is used to bus the output signalsfrom the column amplifiers 15 to the ROIC output.

To simplify the multiplexing process and system interface, the ROICcontains digital logic circuitry to generate the signals required tocontrol the row and column address switches. FIGS. 12 and 13 show animplementation of logic circuitry capable of generating the addressingsignals for the row and column address switches. In each case a chain ofD-flip flops 21 is used to shift an addressing signal through the rowand column enables. The multiplexing process is performed by enabling arow and then sequentially enabling the column selects.

The addressing synchronization signals RowSync and ColumnSync (FIGS. 12and 13 respectively) are inputs to the ROIC or are generated by on-ROIClogic. These signals are driven to the first D-flop's 21 "D" terminaland inverted by inverter 20 to the "D-bar" terminal. Row and columnclocks are used to shift the sync pulses down the shift registers. ANDgates 22 are used to decode a unique addressing state for each of theRow Enable and Column Enable outputs.

FIG. 14 illustrates the placement of the components to provide a ROICfor an 8×8 array of microbolometer detectors. The array of unit cells,column amplifiers, a column multiplexer 25 and a row 26 multiplexer areintegrated on to a single ROIC silicon die. The microbolometer array isconstructed on top of the unit cell array. In addition to the circuitspreviously described, bias generation and timing control circuitry 24and an output amplifier 27 are included. The ROIC provides criticalinterfaces for both the microbolometer detector array and the externalsystem.

Microbolometer Non-Uniformity Correction

When uniform electrical bias and incident infrared radiation are appliedto an array of microbolometer detectors, differences in detectorresponse will occur. As noted above, this is commonly referred to asspatial non-uniformity, and is due to the distribution in values of anumber of critical performance characteristics of the microbolometerdetectors, a natural result of the microbolometer fabrication process.The characteristics contributing to detector non-uniformity include thedetectors' infrared radiation absorption coefficient, resistance,temperature coefficient of resistance (TCR), heat capacity, and thermalconductivity.

The magnitude of the response non-uniformity can be substantially largerthan the magnitude of the response due to the incident infraredradiation. The resulting ROIC output signal is difficult to process asit requires system interface electronics having a very high dynamicrange. In order to achieve an output signal dominated by the responsedue to incident infrared radiation, processing to correct for detectornon-uniformity is required.

FIG. 15 shows a conventional method to correct for microbolometernon-uniformities. A single detector signal path is shown for simplicity.Here a uniform bias 11 is applied to all of the microbolometer arraydetectors 3. Current from the microbolometer is integrated by theintegration stage 28. Offset 29 and gain 30 correction functions areshown at the output of the integration stage. The offset and gaincorrections are addition and multiplication functions, respectively.These functions together are commonly referred to as two-pointcorrection. It is possible for the offset and gain corrections to beimplemented as a part of the integration stage or after the integrationstage, on or off of the ROIC, and either in analog or digital form. Inaddition, the offset correction can be performed either before or afterthe gain correction.

FIGS. 16A, 16B and 16C illustrate the traditional two-point correctiontechnique. The graph in FIG. 16A shows the transfer function for twodetectors having different responses to the same optical (infrared)stimulus. Qmin and Qmax are the maximum and minimum anticipated levelsof infrared radiation, respectively. The graph in FIG. 16B shows theapplication of offset correction. Offset correction coefficients areacquired at Qmin for this operation. The graph in FIG. 16C shows theapplication of both offset and gain correction. Here gain coefficientsare calculated for signal response between Qmin and Qmax. For arrayswith linear signal transfer functions, this technique can provide a highdegree of correction and produce an image that is pleasing to the eye.However, due to the high sensitivity of microbolometer arrays tosubstrate temperature, traditional two-point correction methods aresuccessful for only a small range of substrate temperatures.

Values for the gain and offset correction terms, or coefficients, arespecific to each microbolometer detector and are generated and storedduring a calibration process. For a constant substrate temperature, twouniform infrared illumination levels (such as Qmin and Qmax) can be usedto acquire the gain and offset correction coefficients. At a substratetemperature T₁ and uniform infrared illumination level Qmin, the signaloutputs of the detectors can be used to derive the offset coefficients.By then changing the uniform infrared illumination to a different levelQmax while maintaining the substrate temperature at T₁, the gaincoefficients can be calculated from the output signals.

The gain and offset correction coefficients generated in this way arenormally stored in a correction coefficient memory. The output signalfrom the sensor is converted to digital form and is processed throughgain and offset (multiply and addition) processes. Correctioncoefficient data are retrieved from the correction coefficient memoryand applied to the output data in the multiply and addition processes.

The two-point correction process is further described in C. G. Bethea etal., "Long Wavelength Infrared 128×128 Al_(x) Ga_(1-x) As/GaAs QuantumWell Infrared Camera and Imaging System", IEEE Transactions On ElectronDevices, Vol. 40, No. 11, November 1993, pp. 1957-1963, which isincorporated herein by reference in its entirety.

The conventional two-point correction process can provide spatialnon-uniformity correction below the theoretical temporal noise of themicrobolometer detectors. This correction technique is only effective,however, for a small range of microbolometer substrate temperaturevariations (on the order of 0.01 degree Kelvin).

FIGS. 17A-17C illustrate graphs showing the number of elements in amicrobolometer array (vertical axis) having a given signal output(horizontal axis) in the presence of uniform input signals (bias andincident optical radiation). FIG. 17A shows the simulated uncorrectedsignal distribution for a microbolometer array. FIG. 17B shows thesimulated resulting distribution after applying a two-point correction.FIG. 17C shows the simulated resulting signal distribution after themicrobolometer substrate temperature has been changed.

It is thus apparent that microbolometer arrays have a large sensitivityto the ROIC substrate temperature. Changes in the substrate temperatureintroduce substantial errors to the non-uniformity correction results.FIG. 18 shows the two-point corrected signal distribution for an arrayof microbolometers as a function of substrate temperature. TemperatureT₁ was used as the substrate temperature for the calibration process.For a substrate temperature equal to T₁, near ideal spatialnon-uniformity correction is achieved. As the substrate temperaturemoves away from T₁ the spatial non-uniformity rapidly increases.

FIG. 19 shows the non-uniformity vs. optical signal for a microbolometerROIC array corrected using the traditional two-point method. Thevertical axis is plotted as sigma/mean of the array output signal, whichis a measure of the spatial non-uniformity of the ROIC array. Qmin andQmax are the optical illumination levels used to generate the two-pointgain and offset correction coefficients. At optical illumination levelsabove Qmax or below Qmin the spatial non-uniformity for the sensordegrades rapidly. Also note that the spatial non-uniformity degradesbetween Qmin and Qmax, but to a limited extent due to nonlinearity.

For a microbolometer sensor array that has been corrected using thetraditional two-point method with substrate temperature at Tnominal (themidpoint between the maximum and minimum expected temperatures), thespatial non-uniformity degrades rapidly when the substrate temperatureis changed from Tnominal. This effect is shown in FIG. 20. Two pointsare shown where the spatial non-uniformity is at a minimum. These are atQmin and Qmax where the substrate is at Tnominal. As the substratetemperature changes, the spatial non-uniformity and sigma/mean degradesrapidly.

Systems utilizing past microbolometer infrared detector technologieshave required cooling systems, vacuum packaging systems and complexprocessing electronics to maintain the substrate temperature within avery tight tolerance (e.g., 0.01 degrees Kelvin). The added cost ofthese systems has impeded the development of a high-volume, low-costcommercial market for infrared imaging systems. A microbolometerinfrared detector array which did not require high tolerance coolingwould have the potential to become the first infrared technology toallow penetration into high-volume, low-cost commercial markets.

SUMMARY OF THE INVENTION

The following disclosure describes an invention that provides a new andgreatly improved method for performing non-uniformity correction onmicrobolometer focal plane arrays. This invention allows a significantreduction in the complexity and cost associated with correcting thespatial non-uniformity of the microbolometer detectors and thus solves asignificant technical problem impeding low-cost production ofmicrobolometer infrared imaging systems.

The first section of this application describes embodiments of a circuitthat incorporates substrate temperature compensation circuitry,providing the ability to perform non-uniformity correction for a rangeof substrate temperatures over 0.1 degree Kelvin (the prior art rangewas 0.005 to 0.025 degree Kelvin).

The first embodiment disclosed is a circuit which includes a common gateamplifier with a substrate temperature compensated load. This circuitprovides substrate temperature compensation for the load impedance andoffset current. The microbolometer array is uniformly biased by a biasgenerator and the source potential of the common gate amplifier. Asecond bias generator provides the common gate amplifier gate bias. Athermally-shorted microbolometer is used as a load for the circuit. Aload bias can be adjusted to optimize the operating point for thecircuit. A second amplifier is used to amplify the signal level at thenode connecting the load resistor and the drain of the common gateamplifier. As a result of the substrate temperature compensationprovided in this circuit, the range of substrate temperature change thatis possible while maintaining the two-point corrected non-uniformitysignal distribution below the microbolometer temporal noise level isincreased by more than an order of magnitude over non-compensatedcircuit approaches.

The second section of this application describes methods by which anon-uniform bias is applied to a microbolometer array, providing theability to perform non-uniformity correction for a range of substratetemperatures over 10 degrees Kelvin. Two methods are described. Onemethod is a bias-correction method that is performed on themicrobolometer detectors during the current integration period (alsoreferred to as the "bias period"). The second method is apre-bias-correction method that is performed on the microbolometerdetectors before the current integration or bias period. With both thebias and pre-bias methods, the microbolometer detector array is biasedsuch that the ratio among the output signals from the individualmicrobolometers remains nearly constant as the substrate temperaturechanges. After the bias or pre-bias correction method has been applied,a standard two-point gain and offset non-uniformity correction is usedto correct the non-uniformities in the properties of the microbolometerdetectors.

The bias-correction method involves the application of a unique voltageor current to each microbolometer detector during the integrationperiod. The pre-bias-correction method involves the application of aunique voltage or current to each microbolometer detector prior to theintegration period. The unique voltage or current may be applied with adigital-to-analog converter or with some other element such as areactive component (e.g., a capacitor) connected in the circuit. Theultimate objective is to transfer a predetermined amount of energy toeach microbolometer detector so as to vary the temperature of thedetector by a predetermined amount. Any circuitry or components thatachieve this purpose are within the scope of this invention.

The application of bias or pre-bias correction, followed by conventionaltwo-point gain and offset correction, provides excellent non-uniformitycorrection over a much wider range of substrate temperatures than waspreviously possible. Either of these correction techniques results in areduction in substrate temperature sensitivity of two to three orders ofmagnitude over previous correction schemes. The third section of thisapplication describes circuits that can be used to implement the biasand pre-bias methods. All of the circuits described also incorporatesubstrate temperature compensation circuitry.

Two embodiments of circuitry using the bias-correction method formicrobolometer infrared detectors are described, including amicrobolometer bias-corrected common gate amplifier, and amicrobolometer bias-corrected transimpedance amplifier, respectively.For each embodiment three versions are shown for the placement of thebias-correction circuitry.

The fourth section of this application describes circuitry forperforming on-ROIC substrate temperature control for microbolometerinfrared focal plane arrays.

In one embodiment, a circuit to perform on-ROIC substrate temperaturecontrol includes a current source for supplying a constant currentthrough a thermally-shorted microbolometer. This circuit is used tosense the substrate temperature. An amplifier drives an on-ROIC resistorto heat the ROIC substrate to a constant temperature. A voltage sourcemay be used to set the desired substrate temperature. A low pass filtermay be used at the input of amplifier to stabilize the thermal controlloop.

The resistive elements required to implement on-ROIC circuitry tocontrol the temperature of the ROIC substrate are the substratetemperature sensor resistor and the heating element resistor. A materialwith a low thermal conductivity, such as silica glass, may be used toprovide a thermal standoff from the surrounding environment.

The fifth section of this application describes circuitry located on theROIC for controlling the offset of the signal generated by eachmicrobolometer in the array.

This is accomplished by providing an adjustable bias across eachthermally-shorted microbolometer. In one embodiment, an adjustable loadbias is provided at the negative terminal of each thermally-shortedmicrobolometer, and the values of the load biases are set such that thevoltage offsets resulting from the microbolometers in the array areequalized. In a second embodiment, an adjustable bias is applied to thegate of a transistor that is connected in series with thethermally-shorted microbolometer, and the values of the gate biases areset to control the size of the currents through the thermally-shortedmicrobolometers and thereby equalize the current offsets of themicrobolometers in the array.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are illustrations of microbolometer symbols.

FIG. 2 is an illustration of the applied voltage method for measuringmicrobolometer resistance.

FIG. 3 is an illustration of the applied current method for measuringmicrobolometer resistance.

FIG. 4 is an illustration of the load resistor method for measuringmicrobolometer resistance.

FIG. 5 is a schematic diagram of a bridge configuration using a shieldedmicrobolometer for measuring microbolometer resistance.

FIG. 6 is a schematic diagram of a single microbolometer with twoterminal interconnection.

FIG. 7 is a perspective view of a single microbolometer detector.

FIG. 8 is a schematic diagram of a three-by-three array of two-terminalbolometers showing nine positive and negative interconnect terminals.

FIG. 9 is an illustration of a three-by-three array of two-terminalmicrobolometers requiring nine positive and nine negative interconnectterminals.

FIG. 10 is a schematic diagram of a one-dimensional multiplexer withthree detectors.

FIG. 11 is a schematic diagram of a two-dimensional array ofmicrobolometers and associated addressing, bias and amplificationcircuitry.

FIG. 12 is a schematic diagram of the logic used to address the RowEnable transistors.

FIG. 13 is a schematic diagram of the logic used to address the ColumnEnable select transistors.

FIG. 14 is an illustration of a ROIC for bolometers.

FIG. 15 is a schematic diagram of the conventional two-point gain andoffset correction process for microbolometer non-uniformity correction.

FIGS. 16A, 16B and 16C illustrate a conventional two-point correctionprocess using analog or digital correction methods.

FIGS. 17A, 17B and 17C are illustrations of the simulated correctionprocess results and substrate temperature sensitivity for uniformapplied microbolometer bias.

FIG. 18 is an illustration of the correction process temperaturestability for uniform applied microbolometer bias.

FIG. 19 shows the result of applying two-point correction to the ROIC atconstant substrate temperature equal to Tnominal.

FIG. 20 is an illustration of substrate uniformity stability as afunction of temperature around the calibration temperature Tnominal.

FIG. 21 is a schematic diagram of a common gate amplifier incorporatingthermally-shorted micro-bolometer load to provide substrate temperaturecompensation.

FIG. 22 is a schematic diagram of a common gate amplifier with substratetemperature compensated load circuit using supply side of themicrobolometer row enable select transistor.

FIG. 23 is a schematic diagram of a common gate amplifier with substratetemperature compensated load circuit using common gate amplifier side ofthe microbolometer row enable select transistor.

FIG. 24 is a schematic diagram of a transimpedance amplifier withsubstrate temperature compensated offset.

FIGS. 25A, 25B and 25C are illustrations of the correction processresults and substrate temperature sensitivity for uniform appliedmicrobolometer bias using substrate temperature compensated circuitconfigurations.

FIG. 26 is a schematic diagram of a bias-corrected microbolometer withtwo-point offset non-uniformity correction.

FIG. 27 is a schematic diagram of a pre-bias-corrected microbolometerwith two-point gain and offset non-uniformity correction.

FIGS. 28A, 28B and 28C are illustrations of the correction processresults and temperature sensitivity for a bias or pre-bias-correctedmicrobolometer detector array.

FIG. 29 is a schematic diagram for a microbolometer bias-correctedcommon gate amplifier with temperature compensated load using gateadjustment of microbolometer bias.

FIG. 30 is a schematic diagram for a microbolometer bias-correctedcommon gate amplifier with temperature compensated load using supplyadjustment of microbolometer bias.

FIG. 31 is a schematic diagram for a microbolometer bias-correctedcommon gate amplifier with temperature compensated load using voltagedivider supply adjustment of microbolometer bias.

FIG. 32 is a schematic diagram for a microbolometer bias-correctedtransimpedance amplifier with substrate temperature compensated offsetusing gate adjustment of microbolometer bias.

FIG. 33 is a preferred circuit implementation of the column amplifiershown above in FIG. 32.

FIG. 34 is a schematic diagram for a microbolometer bias-correctedtransimpedance amplifier with substrate temperature compensated offsetusing supply adjustment of microbolometer bias.

FIG. 35 is a schematic diagram for a microbolometer bias-correctedtransimpedance amplifier with substrate temperature compensated offsetusing a voltage divider supply adjustment of microbolometer bias.

FIG. 36 is a schematic diagram of a one dimensional multiplexer withthree detectors showing application of the bias-correction method at thedetector bias supply.

FIG. 37 is a schematic diagram of a two dimensional three-by-threedetector multiplexer showing application of the bias-correction methodat the column amplifier input.

FIG. 38 is a schematic diagram of a two dimensional three-by-threedetector multiplexer showing application of the bias-correction methodat the reference potential for each column amplifier input.

FIG. 39 is a schematic diagram of a two dimensional three-by-threedetector multiplexer showing application of the bias-correction methodat the detector bias supply for each column.

FIG. 40 is an illustration of the bias-correction circuitry components.

FIG. 41 is a schematic diagram of the functional elements for the biascompensation circuit block.

FIG. 42 is an illustration of the timing for the digital-to-analogconverter (DAC) correction data load process.

FIG. 43 is an illustration of the timing for the ROIC data load process.

FIG. 44 is an illustration of a microbolometer ROIC with bias-correctioncircuitry and interface system electronics.

FIGS. 45A, 45B, 45C and 45D illustrate a bias-corrected two-pointcompensation process using 12-bit coefficients.

FIG. 46 is a block diagram of bias-corrected three-point non-uniformitycompensation technique.

FIGS. 47A and 47B illustrate the bias-corrected three-pointnon-uniformity compensation technique. FIG. 47A shows two detectortransfer functions prior to compensation. FIG. 47B shows mean gain atthe substrate temperatures Tmin and Tmax.

FIGS. 48A and 48B shows an illustration of the bias-correctedthree-point non-uniformity compensation technique. FIG. 48A shows twodetectors transfer functions after bias-correction compensation. FIG.48B shows the two detectors after bias-correction compensation andtraditional gain and offset processing.

FIG. 49 is an illustration of resulting spatial non-uniformity forbias-corrected three-point calibration.

FIGS. 50A and 50B show a flow diagram for bias compensated gaintwo-point correction coefficient generation.

FIGS. 51A and 51B show a flow diagram for bias-corrected three-pointcompensated gain ratio correction coefficient generation.

FIG. 52 is a schematic diagram for a circuit to perform on-ROICsubstrate temperature control.

FIG. 53 is a schematic diagram for a microbolometer focal plane arraywith on-ROIC thermal control circuitry.

FIG. 54 is a schematic diagram of a circuit for applying an adjustableload bias to the thermally-shorted microbolometer to provide anadjustable voltage offset.

FIG. 55 is a schematic diagram of a circuit for applying an adjustablebias to a transistor in series with the thermally-shorted microbolometerto provide an adjustable current offset.

DETAILED DESCRIPTION OF THE INVENTION

The first section of this detailed description describes circuitembodiments that incorporate substrate temperature compensationcircuitry, providing the ability to perform non-uniformity correctionfor a range of substrate temperatures over 0.1 degree Kelvin.

The second section of this detailed description describes methods bywhich a non-uniform bias is applied to a microbolometer array, providingthe ability to perform non-uniformity correction for a range ofsubstrate temperatures over 10 degrees Kelvin.

The third section of this detailed description describes circuitembodiments that apply a non-uniform bias to a microbolometer array andincorporate substrate temperature compensation circuitry, providing theability to perform non-uniformity correction for a range of substratetemperatures over 10 degrees Kelvin.

The fourth section of this detailed description describes circuitry toperform on-ROIC substrate temperature control for microbolometerinfrared focal plane arrays.

The fifth section of this detailed description describes on-ROICcircuitry for correcting the offset of the voltage or current output ofeach microbolometer detector in the array.

I. Embodiments of Circuits that Incorporate Substrate TemperatureCompensation Circuitry

The first circuit disclosed includes a common gate amplifier with asubstrate temperature compensated load, as shown in FIG. 21. Thiscircuit provides substrate temperature compensation for the loadimpedance and offset current. A thermally isolated detectormicrobolometer 3 is uniformly biased by a bias generator 11 and thesource potential of a common gate amplifier or MOSFET 31. A second biasgenerator 33 provides the gate bias for the common gate amplifier 31. Athermally-shorted microbolometer 2 is used as a substrate temperaturecompensated load for the circuit. Bias Vload is adjusted to optimize theoperating point for the circuit by setting Vout at a desired pointwithin a range of output voltages. A second amplifier 32 is used toamplify the signal level at the node connecting the load resistor(microbolometer 2) and the drain of the common gate amplifier 31. Thecommon gate amplifier 31 shown is a PMOS transistor; however, an NMOSimplementation is also possible.

To implement this circuit in an array configuration, part of the circuitis placed in the unit cell and part in the column amplifier as shown inFIGS. 10 and 11. In addition, a select transistor is required to supportthe row addressing for multiplexing. FIG. 22 shows placement of a selecttransistor 10 on the supply side of the microbolometer 3. FIG. 23 showsplacement of select transistor 10 on the common gate amplifier side ofthe microbolometer 3. For these circuits the select transistor 10 andthe microbolometer 3 are placed in the unit cell. The common gateamplifier 31, microbolometer 2, and amplifier 32 are placed in thecolumn amplifier.

The incident radiation 1 heats the microbolometer 3, which lowers theresistance of the microbolometer 3. During the bias integration periodthe current through microbolometer 3 is determined by the voltage acrossand resistance of microbolometer 3. The voltage across microbolometer 3is equal to the difference between the voltage of the bias generator 11and the source potential of common gate amplifier 31. Higher levels ofincident radiation 1 cause the temperature of microbolometer 3 to riseand the resistance of microbolometer 3 to fall, and therefore a largercurrent flows into the source of common gate amplifier 31. Since thesource and drain currents of common gate amplifier 31 are approximatelyequal, the same current flows through thermally-shorted microbolometer2. The voltage across microbolometer 2 is amplified by the voltageamplifier 32. Thus, increased levels of incident radiation 1 result in ahigher current through and voltage across microbolometer 2, and thisvoltage change is amplified by voltage amplifier 32 to generate theoutput voltage Vout.

The circuit shown in FIG. 21 has a limited response to changes in thesubstrate temperature as a result of temperature tracking betweenmicrobolometer 3 and thermally-shorted microbolometer 2. To a firstorder, microbolometer 3 and thermally-shorted microbolometer 2 trackchanges in the substrate temperature. Since essentially the same currentflows through microbolometers 2 and 3 and the resistance ofmicrobolometers 2 and 3 track substrate temperature changes, therespective voltages across microbolometers 2 and 3 do not changeappreciably with changes in the substrate temperature.

The bias Vload is used to change the voltage across thermally-shortedmicrobolometer 2 and thereby adjust the potential at the input ofvoltage amplifier 32. In this manner the operation point of voltageamplifier 32 is adjusted.

The circuits shown in FIGS. 21-23 have the advantage that substratetemperature fluctuations are compensated by the load to the first order.This is due to the fact that the temperature (and therefore resistances)of the active microbolometer 3 and the thermally-shorted microbolometer2 both track changes in the substrate temperature. A second advantage ofthese circuits is that signal gain can be established at the amplifier32 input by setting the resistance value of the load microbolometer 2 toa desired ratio to the active detector microbolometer 3. And the secondbias generator 33 is used to drive the gate of the common gate amplifier31 in order to provide a variable bias for the microbolometer 3.

A second microbolometer amplifier circuit, including a transimpedanceamplifier with substrate temperature compensated offset, is shown inFIG. 24. Here current from the thermally isolated detectormicrobolometer 3 is amplified and integrated by the amplifier 34 andcapacitive feedback circuit 35, which together form a transimpedanceamplifier 34A. A thermally-shorted microbolometer 2 is used to provide asubstrate temperature compensated offset current for the circuit.Microbolometer 3 responds as described above to changes in the incidentradiation 1. In this case, however, the drain current of the common gateamplifier 31 flows into the inverting input of the transimpedanceamplifier 34A. The voltage across thermally-shorted microbolometer 2generates a second current which flows out of the inverting input of thetransimpedance amplifier 34A. Transimpedance amplifier 34A generates anoutput voltage that is dependent on the difference between these twocurrents.

An increased level of radiation 1 produces a larger current flowingthrough common gate amplifier 31 and into the inverting input oftransimpedance amplifier 34A. Since the impedance at the inverting inputof the transimpedance amplifier is low, the voltage acrossthermally-shorted microbolometer 2 is unaffected by the change incurrent from microbolometer 3 and common gate amplifier 31. Similarly,the current through microbolometer 2 is, to the first order, unaffectedby the change in current from microbolometer 3 and common gate amplifier31. Since the current through thermally-shorted microbolometer 2 is thesame as the current flowing out of the inverting input of transimpedanceamplifier 34A, the difference current at the inverting input of thetransimpedance amplifier is a function only of changes in the currentthrough microbolometer 3.

Since the resistances of microbolometers 2 and 3 track changes in thesubstrate temperature, as described above, and the voltages acrossmicrobolmeters 2 and 3 are constant, changes in the substratetemperature result in no net change in the difference current at theinverting input of transimpedance amplifier 34A.

Bias Vload is used to adjust the offset current from microbolometer 2and can be used to set the operational point of the transimpedanceamplifier 34A.

As in circuits shown in FIGS. 22 and 23, the row select transistors inthe unit cell can be located on either side of the detectormicrobolometer 3. And the second bias generator 33 is used to drive thegate of the common gate amplifier 31 in order to provide a variable biasfor the microbolometer 3.

FIGS. 25A, 25B and 25C illustrate the results of performing a two-pointcorrection using the substrate temperature compensated circuits shown inFIGS. 21-24. FIGS. 25A-25C correspond to FIGS. 17A-17C, respectively. Asin FIGS. 17A-17C, the non-uniformity distribution is corrected using atwo-point gain and offset correction algorithm. The temperature of thesubstrate is then changed and the spatial non-uniformity rapidlyincreases, as shown in FIG. 25C. Nonetheless, due to the substratetemperature compensation in these circuits, the range of substratetemperature change that is possible while maintaining the two-pointcorrected non-uniformity signal distribution below the microbolometertemporal noise level is increased by more than an order of magnitudeover non-compensated circuit approaches.

II. Embodiments of Methods which Apply Non-uniform Bias to aMicrobolometer Array

In the previously discussed circuits, a uniform bias is applied to themicrobolometer array. In these circuits all array detectors are biasedusing a single bias supply value. During the bias period, amicrobolometer detector significantly increases in temperature for smallduty cycles as:

    Trise=Pbias*Time/ThermalMass

where Pbias is the electrical power input to generate a detector outputsignal, Time is the bias period and ThermalMass is the heat energyrequired to heat the microbolometer detector one degree Kelvin.

In order to maximize the signal-to-noise ratio from the detector, thesignal must be frequency band-limited. The most common and effectivetechnique of band limiting is signal integration over the bias period.The microbolometer's resistance changes during the bias period due tothe change in temperature of the microbolometer detector. Theintegration process in effect averages the response during this periodto yield a single integrated value.

The microbolometer detector resistance, Rdet(T), can be modeled as:

    Rdet(T)=Ro*exp(alpha*To.sup.2 *(To.sup.-1 -T.sup.-1))

where Ro is the microbolometer resistance at a standard temperature, To,(usually 300 degrees Kelvin), alpha is the temperature coefficient ofresistance (TCR) at To, and T is the substrate temperature.

The integrated current (charge) flowing out of a fixed voltage bias,Vbias, over an integration/bias period, Tint, is then: ##EQU3## Similarexpressions can be derived for current bias or resistive bias withintegration of output voltage.

If a uniform bias is applied to all detectors in a non-uniformmicrobolometer array, the ratio of the response of their integratedoutputs to the optical signal will vary significantly over even a smallrange in substrate temperature. According to this invention, uniquecompensating biases are applied to the microbolometer array detectors.Two methods of accomplishing this will be described.

A. Bias-correction Method

According to the bias-correction method, a unique bias amplitude isapplied to each detector during the integration period to supportuniformity correction.

The bias-correction method can be implemented as an adjustable voltage,current, or load bias that is applied to the microbolometer detectorsduring the integration (measurement) period. FIG. 26 conceptuallyillustrates an adjustable voltage implementation of the bias-correctionmethod. Here, the bias-correction value is applied during theintegration period of the microbolometer detector using an adjustablevoltage source 36. The bias-correction value is controlled by the outputof a digital-to-analog converter (DAC) (not shown). The adjustablebolometer bias may be used to correct the optical gain of the signal foruniform output at a particular substrate temperature in conjunction withsingle-point offset correction 30 to remove residual fixed offsets.

Non-uniformity correction can be achieved over a very wide range ofsubstrate temperatures when the bias-correction method is used to biasthe microbolometer detector array such that the ratio of the response ofthe individual integrated microbolometer output signals to the meanoptical signal response of the array remains nearly constant as thesubstrate temperature changes. After the bias-correction method has beenapplied, a standard two-point gain 29 and offset 30 non-uniformitycorrection is used. With this method, the bias-correction is performedon the ROIC prior to the integration process.

B. Pre-Bias-correction Method

According to the pre-bias-correction method a unique bias amplitude isapplied to each detector during a selected time interval prior to theintegration period to support uniformity correction.

The pre-bias-correction method can be implemented as an adjustablevoltage, current, or load bias that is applied to the microbolometerdetectors prior to the integration period. FIG. 27 conceptuallyillustrates an adjustable voltage implementation of the bias-correctionmethod. Here, the pre-bias-correction value 36 is applied to themicrobolometer detector through switch 37 during a specified timeinterval (the "pre-bias period") prior to the integration period. Switch37 is closed and switch 10 is open during the pre-bias period; theconditions of the switches are reversed during the integration period.

A uniform bias 11 is applied to all microbolometer detectors during theintegration period. The adjustable bolometer pre-bias may be used tocorrect the optical gain of the signal for uniform output at aparticular substrate temperature in conjunction with single-point offsetcorrection 30 to remove residual fixed offsets.

Non-uniformity correction can be achieved over a very wide range ofsubstrate temperatures when the pre-bias-correction method is used topre-bias the microbolometer detector array such that the ratio of theresponse of the individual integrated microbolometer output signals tothe mean optical signal response of the array remains nearly constant asthe substrate temperature changes. The pre-bias will increase ordecrease the output signal of the integrating amplifier 28 by a selectedamount. After the pre-bias-correction method has been applied, astandard two-point gain 29 and offset 30 non-uniformity correction isused. The pre-bias-correction must be performed on the ROIC prior to theintegration process.

The application of the bias or pre-bias-correction methods followed byconventional two-point gain and offset correction provides excellentnon-uniformity correction over a much wider range of substratetemperatures than was previously possible. Either of these correctiontechniques results in a reduction in substrate temperature sensitivityof two to three orders of magnitude over previous correction schemes.FIGS. 28A, 28B and 28C illustrate the simulated reduced sensitivity tosubstrate temperature changes for the bias or pre-bias-correctionmethods. FIG. 28A shows the signal distribution before any correction,FIG. 28B shows the signal distribution after two-point correction, andFIG. 28C shows the signal distribution after two-point and bias orpre-bias-correction methods.

Applying Correction Coefficients to two Circuit Implementations for theBias-correction Method

The specific implementation of two circuit approaches using thebias-correction method for microbolometer infrared detectors aredescribed here, the microbolometer bias-corrected common gate amplifierwith temperature compensated load, and the microbolometer bias-correctedtransimpedance amplifier with substrate temperature compensated offset.For each circuit approach three versions are shown for the placement andimplementation of the bias-correction circuitry.

For simplicity the following circuit figures are drawn without the rowselect transistors shown. As in the common gate temperature compensatedload circuit (FIGS. 22 and 23), the row select transistors in the unitcell can be located on either side of the microbolometer detector.

Microbolometer Bias-corrected Common Gate Amplifier with TemperatureCompensated Load

The microbolometer bias-corrected common gate amplifier with temperaturecompensated load provides substrate temperature compensation for theload impedance and offset current as well as bias-correction for themicrobolometer detector. Three implementations for the bias-correctionportion of this circuit are described.

FIG. 29 shows a schematic of a circuit for the microbolometerbias-corrected common gate amplifier with temperature compensated load,using a gate adjustment configuration to vary the microbolometer bias.(It will be noted that FIG. 29 is identical to FIG. 21, except that aDAC 36 has been substituted for the bias source 33.) The microbolometer3 is biased by the voltage source 11 and the source potential of thecommon gate amplifier 31. A digital-to-analog converter (DAC) 36 is usedto drive the gate of the common gate amplifier in order to provide avariable bias for the microbolometer 3. A thermally-shortedmicrobolometer 2 is biased by Vload and is used as a load for thecircuit. A second amplifier 32 is used to amplify the signal at the nodebetween microbolometer 2 and the drain of the common gate amplifier 31.

FIG. 30 shows a schematic of another circuit for the microbolometerbias-corrected common gate amplifier with temperature compensated load,using a supply adjustment configuration to vary the microbolometer bias.The microbolometer 3 is biased by the DAC 36 and the source potential ofthe common gate amplifier 31. A fixed voltage source 33 is used to drivethe gate of the common gate amplifier. The DAC provides a variable biasfor the microbolometer 3, with n bits input for gain correction. Athermally-shorted microbolometer 2 is biased by Vload and is used as aload for the circuit. A second amplifier 32 is used to amplify thesignal at the node between microbolometer 2 and the drain of the commongate amplifier 31.

FIG. 31 shows the schematic of yet another circuit for themicrobolometer bias-corrected common gate amplifier with temperaturecompensated load, using a voltage divider and DAC 36 to adjust themicrobolometer bias. The microbolometer 3 is biased by a voltagedivider, formed by resistors 6 and 7, driven by DAC 36, and the sourcepotential of the common gate amplifier 31. A fixed voltage source 33 isused to drive the gate of the common gate amplifier 31. The DAC 36provides a variable bias for the microbolometer detector. Athermally-shorted microbolometer 2 is used as a load for the circuit. Asecond amplifier 32 is used to amplify the signal at the node betweenmicrobolometer 2 and the drain of the common gate amplifier 31.

Microbolometer Bias-corrected Transimpedance Amplifier with SubstrateTemperature Compensated Offset

The microbolometer bias-corrected transimpedance amplifier withsubstrate temperature compensated offset provides substrate temperaturecompensation for the offset current as well as bias-correction for themicrobolometer detector. Three implementations for the bias-correctionportion of this circuit are described.

FIG. 32 shows a schematic of a circuit for the microbolometerbias-corrected transimpedance amplifier with substrate temperaturecompensated offset, using a gate adjustment configuration to vary themicrobolometer bias. (Note that FIG. 32 is identical to FIG. 22, exceptthat DAC 36 has been substituted for bias source 33.) The microbolometer3 is biased by the voltage source 11 and the source potential of thecommon gate amplifier 31. A DAC 36 is used to drive the gate of thecommon gate amplifier in order to provide variable bias for themicrobolometer detector. A thermally-shorted microbolometer 2 is used togenerate a compensated offset current for the circuit. A transimpedanceamplifier with an amplifier 34 and a feedback circuit 35 is used toamplify and integrate the signal at the node between microbolometer 2and the drain of the common gate amplifier. The component Z in thecapacitive feedback circuit 35 can be implemented as a switchedcapacitor to allow integration.

FIG. 33 provides a detailed, preferred circuit implementation for thefunctional diagram shown in FIG. 32. Here, a folded cascodeimplementation for a CMOS differential amplifier is used in a capacitivetransimpedance amplifier configuration to implement amplifier 34 andfeedback circuit 35 from FIG. 32. The row enable signal 16 is shownbiasing the gate of the row enable select p-channel transistor 10. Themicrobolometer detector element 3 is shown biased by DETCOM, which isenabled by the row enable transistor 10, and the source potential of thecommon gate p-channel amplifier 31. The controlling bias for the gate of31 is supplied from a conventional CMOS DAC (not shown) through asubstrate temperature compensated divider network using thermallyshorted microbolometers 2a and 2b. A second conventional DAC (not shown)is used to supply a bias to an offset network using thermally shortedmicrobolometers 2c and 2d for the control of offset current into thecharge integrating amplifier feedback circuit 35, which can bedischarged by application of RST to the gate of a shorting transistor.

FIG. 34 shows a schematic of a circuit for the microbolometerbias-corrected transimpedance amplifier with substrate temperaturecompensated offset, using a supply adjustment configuration to vary themicrobolometer bias. The microbolometer 3 is biased by DAC 36 and thesource potential of the common gate amplifier 31. A fixed voltage source33 is used to drive the gate of the common gate amplifier 31. The DAC 36provides a variable bias for the microbolometer detector. Athermally-shorted microbolometer 2 is used to generate a compensatedoffset current for the circuit. A transimpedance amplifier including anamplifier 34 and feedback circuit 35 is used to amplify and integratethe signal at the node between microbolometer 2 and the drain of thecommon gate amplifier 31. The component Z in feedback circuit 35 can beimplemented as a switched capacitor to allow integration. In this case,by changing the size of the capacitor, the gain for the integrator canbe varied.

FIG. 35 shows a schematic of a circuit for the microbolometerbias-corrected transimpedance amplifier with substrate temperaturecompensated offset, using a voltage divider supply adjustment ofmicrobolometer bias. The microbolometer 3 is biased by a voltagedivider, formed of resistors 6A and 6B, driven by DAC 36, and the sourcepotential of the common gate amplifier 31. A fixed voltage source 33 isused to drive the gate of the common gate amplifier. The DAC provides avariable bias for the microbolometer 3. A thermally-shortedmicrobolometer 2 is used to generate a compensated offset current forthe circuit. A transimpedance amplifier including an amplifier 34 and afeedback circuit 35 is used to amplify and/or integrate the signal atthe node between the load resistor and the drain of the common gateamplifier 31. The transimpedance amplifier feedback component Z can beimplemented as a switched capacitor to allow integration.

The output voltage could be taken at other points in the circuits shownin FIGS. 29-35, for example, across the capacitive feedback circuit 35.

III. Embodiments of Circuits which Apply a Non-uniform Bias to aMicrobolometer Array and Incorporate Substrate Temperature CompensationCircuitry

In order to implement the bias-correction method an adjustable bias mustbe supplied to each of the microbolometer array detectors. FIG. 36 showsthe implementation of a single column of three detectors. Here themicrobolometer bias is modified by three adjustable voltage sourcesshown here as v₁, v₂, V₃. These voltage sources are placed in seriesbetween the detector bias supply 11 and the unit cell microbolometerselect transistor 10. The current through each microbolometer detectorwill be sequentially sampled for integration by the column amplifier 15.Column amplifier 15 has an inherent sample-and-hold capability such thatthe output of amplifier 12 will hold a fixed voltage after the currentstops flowing and the integration process stops. The order of sequencingand the time period of each sample is determined by the sequencing andduration of the active period of each row enable signal.

A two-dimensional three-by-three detector multiplexer with animplementation of the bias-correction method is shown in FIG. 37. Herebias sources v₁, v₂, and v₃ are shown in series with the inputs of thecolumn amplifiers 15. Each column amplifier 15 has an inherentsample-and-hold capability such that its output of amplifier 12 willhold a fixed voltage after the current stops flowing and the integrationprocess stops. During each bias period, the current through eachmicrobolometer detector 3 in a given row will be sequentially sampledfor integration by the corresponding column amplifier 15. The order ofsequencing and the time period of each sample is determined by thesequencing and duration of the active period of each row enable signal.Values for the bias sources v₁, v₂, and v₃ are adjusted for each row ofdetectors 3 thus providing unique bias values for each microbolometerdetector. Individual column amplifiers 15 shown in FIG. 37 are selectedby the column enable signal 19 that controls the column switch 18. Anoutput line 17 common to all columns is used to bus the output signalsfrom the column amplifiers 15 to the ROIC output.

A second possible configuration for implementing the bias-correctionmethod is shown in FIG. 38. Here bias sources v₁, v₂, and v₃ are shownbeing applied to the ground reference potential at the non-invertinginput of each column amplifier 15. Values for these bias sources areadjusted for each row of detectors thus providing unique bias values foreach microbolometer detector 3.

A third possible configuration for implementing the bias-correctionmethod is shown in FIG. 39. Here bias sources v₁, v₂, and v₃ are shownbeing applied separately to the detector bias supply for each column.Values for these bias sources are adjusted for each row of detectorsthus providing unique bias values for each microbolometer detector 3.

It will be understood that any of the circuits shown in FIGS. 29-35 canbe incorporated into the two-dimensional arrays shown in FIGS. 37-39 byplacing the components at various locations in the array.

FIG. 40 shows the incorporation of the bias-correction circuitry 39 ontoan ROIC 40. The exploded view of the bias-correction circuit block showsthe four major components: the address control shift register 41, thedata register 42, the data latches 43 and the column digital-to-analogconverters (DAC's) 44. The data latches 43 drive the digital input tocolumn DACs 44. A conventional CMOS DAC architecture, such as describedin a National Semiconductor CMOS Databook, is utilized to provide thecolumn DACs 44. In other embodiments, a single DAC may be shared by morethan one column so that there are fewer DACs than columns.

FIG. 41 illustrates the function of these blocks. The address shiftregister 41 is implemented as a D flip-flop serial register with input"Sync" and controlling clock "Clk". An active or enabling state is inputby the "Sync" and clocked down the shift register 41 by "Clk". The dataregister 42 is comprised of n-bit latches where "n" is the number of DACdata bits. The total number of n-bit latches in the data register 42 isequal to the number of columns in the detector array. The address shiftregister 41 sequentially enables and latches each of the data registerlatches 42 at a unique time allowing unique DAC correction coefficientdata to be loaded into each data register 42 location. Once the dataregister 42 is loaded, a "Line Load" clock is used to transfer the datafrom the data register 42 to the data latches 43. The data latches 43drive the digital input to column DAC's 44. This data register and datalatch configuration is similar to a "master-slave" latch that canmaintain a stable "slave" output of correction coefficients to thecolumn DACs while new data is loaded into the "master", data register42, input for the next row of correction coefficients.

FIG. 42 shows the timing for the bias-correction circuit 39 shown inFIGS. 40 and 41. "Clk" is the clock to the address shift register 41."Sync" is the input to the address shift register 41. "Q1" is the outputof the column 1 enable of the address shift register 41, "Q2" is theoutput of the column 2 enable of the address shift register 41, and soon. "Data" represents the data inputs of the data registers 42. Notethat a decoded binary or gray code count scheme may also be used toaddress the data register latches 42 for storage of the digitalcorrection data.

A serial digital data stream of n-bit words is supplied to the readoutintegrated circuit to be stored in the DAC data latches. FIG. 43 showsthe data load timing in relationship to the overall ROIC timing. A"Frame Sync" pulse is provided to establish the frame synchronization."Line Sync" pulses are supplied at the start of each line time. A masterclock, "Clk," is used to drive the array shift registers. During a linereadout, column outputs are multiplexed to the ROIC output. At the timea particular column appears at the ROIC output, data is loaded into thedata register for that column, to be applied during the subsequent rowsignal integration time.

As noted above, the circuitry shown in FIG. 41 is a master-slaveimplementation of a digital parallel/serial data interface. Many otherknown implementations may be used for loading the correctioncoefficients into the DACs.

The DAC data words used to generate the bias sources v₁, v₂, and v₃ aregenerated off the sensor in the system electronics. FIG. 44 illustratesthe associated system electronics. The readout integrated circuit 40 isshown in the upper left of this figure. Output signals from the ROIC 40are digitized by an analog-to-digital converter 50 which may be locatedon or off the ROIC. The converted digital data is input to a digitalframe store memory 51. The data in the frame memory 51 is then availablefor the system imaging electronics 52 and the system's data processor55.

The generation of bias-correction data words (or correctioncoefficients) is accomplished by the data processor 55 using acorrection algorithm. Data processor 55 sequences ROIC stimulus, theacquisition of ROIC data, and the calculation of correctioncoefficients. Data processor 55 then loads the correction coefficientsto the correction coefficient memory 57. Data register load circuitry 53interfacing to the correction coefficient memory 57 is used to load thecorrection data into the bias correction circuitry 39 on the ROIC 40 byproviding valid data to the serial data bus interface at the time thatthe data register 42 is latched by the shift register 41. FIG. 44 showsa 12-bit implementation for the data paths in the system.

The generation of the bias compensation correction coefficients isaccomplished by correction algorithms processed by data processor 55.These algorithms and the data processor 55 sequence a calibrationstimulus and the acquisition of the required frames of data. Dataprocessor 55 then calculates the correction coefficients and loads thecoefficients to the correction coefficient memory 57. Circuitryinterfacing to the correction coefficient memory is used to load thecorrection data onto the ROIC 40.

Numerous linear and recursive methods may be used to generate thecorrection coefficients. The simplest method is a linear incrementalmethod. For this method each column DAC is set to the same valuestarting at the lowest value and is then stepped to each higher value byincrementing the DAC count by the least significant bit. At each DACcount four frames of data are acquired from the ROIC by the dataprocessor. Two of the four frames are taken at the lower limit of thesubstrate temperature range (Tmin) at two different optical illuminationlevels (Qmax and Qmin). The two remaining frames are taken at thehighest limit of the substrate temperature range (Tmax) at the same twooptical illumination levels. The substrate temperatures are chosen toreflect the substrate temperature limits that a given system willgenerate for the ROIC and the optical stimulus levels are chosen toreflect the optical scene illumination levels. While the ROIC is at thefirst substrate temperature (Tmin), the DAC values are incremented andoutput data is acquired for the two optical illumination levels. Oncethe output frame or image data has been acquired for all DAC values atthe first substrate temperature, and for the two optical illuminationlevels, the process is repeated at the second substrate temperature(Tmax). The acquired data is temporarily stored in memory or on diskstorage media. After the frames of data have been acquired, dataprocessor 55 calculates the optical gain at each substrate temperaturelevel for each array element and at each DAC setting. The optical gainis equal to:

    G=(VQmax-VQmin)/(Qmax-Qmin)

where VQmax and VQmin are the outputs at the incident radiation levelsof Qmax and Qmin, respectively. The DAC settings are then chosen suchthat, for each array element, the DAC setting generates the same elementoptical gain as a ratio to the array mean optical gain at eachcalibration temperature. That is to say ##EQU4## where G1(Tmax) . . .Gn(Tmax) and G1(Tmin) . . . Gn(Tmin) are the optical gains of each ofthe individual detectors at the first and second substrate temperatures,respectively, and Gm(Tmax) and Gm(Tmin) are the mean optical gains ofall detectors in the array at the first and second substratetemperatures.

Solving the above equations for Gm(Tmax)/Gm(Tmin) yields: ##EQU5##

The DAC settings determined by this process are then stored and loadedas correction coefficients into the correction coefficient memory 57. Atthis point the intermediate data can be discarded.

It is important to recognize that the optical gains for the correctedarray elements at the high and low temperatures are not all required tobe the same. Rather, the optical gain ratios of the various detectorsbetween the substrate temperatures are to be stabilized and equalized tothe mean optical gain ratio between the substrate temperatures. Once theoptical gain ratios are stabilized, a standard two-point correction canbe applied to perform the final gain and offset correction as shown inFIGS. 26 and 27 above.

Although the linear method is effective it requires a large amount ofsystems processor memory to store the intermediate output data (i.e.,the output level at each substrate temperature, each level of incidentradiation, and each DAC setting) during calibration. For this reason itis desirable to use recursive methods for performing the calibration.Here the optical gain is measured at two different substratetemperatures. The DAC coefficient values are then iterated and again theoptical gain is measured at each temperature. The gain ratios areanalyzed to determine if they improved or degraded and the DACcoefficients are adjusted accordingly. It is desired that, for eachelement, the ratio of the optical gain to the mean optical gain remainconstant at each substrate temperature. The process is iterated using asuccessive approximation until the optical gain ratio to the mean gainis stabilized at both substrate temperatures.

The preferred method for the generation of the correction coefficientsis one where both linear and recursive methods are used to establish thecorrection coefficients to stabilize the gains. Here recursive methodscan be used to minimize the amount of memory required to support thesensor system and linear methods can be used to fine adjust thecalibration.

Comparison of the Traditional and New Microbolometer Bias-correctionMethods

As described above, previous methods for performing non-uniformitycorrection use a two-point compensation technique. The two-pointcompensation technique corrects for offset and gain transfer functionerrors and can be implemented on or off of the ROIC.

The bias-correction method may be used to replace the gain correction inthe standard two-point correction process for microbolometer detectors.FIGS. 45A, 45B, 45C and 45D illustrate the processing of two detectorsfor gain and offset correction using the bias-correction method. Thegraph in FIG. 45A shows the uncorrected detector transfer functions attemperatures Tmin and Tmax. Bias-correction is applied at the detectorelements and the gains of the detectors are normalized, as shown in thegraph of FIG. 45B. Finally, offset is corrected as shown in the graph ofFIG. 45C. Offset correction can be performed on or off the ROIC. It ispreferred to perform this function on the ROIC. One advantage over thetraditional two-point method is that the bias-correction for gainadjustment sets the dissipated power per array element more uniformlyand reduces the substrate temperature sensitivity of the array spatialnon-uniformity.

To greatly improve the uniformity compensation with changes in substratetemperature the bias-corrected three-point method can be applied. Theapplication of the bias-correction to the microbolometer array generatesa condition wherein the output signal is temperature compensated andcompatible with traditional two-point offset and gain correction. FIG.46 shows the block diagram for this method. The first step is toselectively bias the microbolometer elements. This is followed by atraditional two-point correction process. The bias-correctioncoefficients are calculated to provide a condition wherein themicrobolometer element optical gains as a ratio to the array meanoptical gain at each calibration temperature are the same.

To illustrate the process of bias-corrected three-point non-uniformitycompensation a series of three-dimensional plots are shown in FIGS. 47A,47B and 48A, 48B. FIGS. 47A and 47B illustrate a graph of two detectors'transfer functions at two substrate temperatures Tmin and Tmax. Thetransfer functions are shown to go between optical illumination levelsQmin to Qmax with the vertical axis showing the signal output. Here theoptical gain of the two detectors is shown to change between the twosubstrate temperatures. FIG. 47B shows the mean gain, (wide gray line),for the two detectors at Tmin and Tmax. The gain ratio compensation stepsets the bias for each microbolometer element such that the ratio of theoptical gain of each detector to the mean optical gain of all thedetectors in the array remains the same at each substrate temperature.FIG. 48A shows the two detectors after bias-correction for gain ratiocompensation. This process results in substrate temperature compensatedspatial non-uniformity that can be corrected using the traditionaltwo-point gain and offset correction. Detector optical gain ratios arecompensated at the substrate temperature minimum and maximum (Tmin andTmax). FIG. 48B shows the result of traditional two-point correctioncompensation applied after bias-correction.

FIG. 49 shows the spatial non-uniformity after applying thebias-corrected three-point correction process. It can be seen that theregion between optical signal levels Qmin to Qmax and substratetemperature range Tmin to Tmax provides a lower level of spatialnon-uniformity.

Preferred Methods For Two- and Three-Point Compensation Using The BiasCorrection Technique

Two flow diagrams are shown to illustrate the procedure to generate thebias-correction coefficients, which are the digital words to be input tothe DACs which constitute the adjustable voltage sources such asadjustable voltage source 36 in FIG. 26. The first diagram is shown inFIGS. 50A and 50B, and it illustrates the calculation of biascoefficients for the bias-corrected two-point compensation method. Twocorrection loops are shown. The first loop, shown in FIG. 50A, sets Tsub(substrate temperature) and Qoptical (optical illumination level) tomid-range. The bias coefficients are then iterated such that the outputsignal is driven to mid-range. The result of this process is thecentering of the dynamic range and the generation of "flat field"correction coefficients.

The second loop, shown in FIG. 50B, generates the bias-correctioncoefficients. The array temperature is set to Tnominal (the midpointbetween Tmin and Tmax) and the illumination level is controlled toprovide the signal Qmin and Qmax as required. The "flat field"coefficients are used as a starting value and the second loop counter ispreset to the number of desired iterations (for example, 64 to 128,depending on the gain non-uniformity magnitude). At each iteration theoptical gain of each detector is measured and the array mean opticalgain is calculated. Bias-correction coefficient values are decrementedfor detectors with an optical gain greater than the mean optical gainand bias-correction coefficient values are incremented for detectorswith an optical gain less than the mean optical gain. The resultingbias-correction coefficients are used to provide gain term correction byapplying a bias compensation to each detector. Bias coefficients arerepresented in the flow diagram as DAC coefficients.

The second diagram is shown in FIGS. 51A and 51B and illustrates thecalculation of bias coefficients for the bias-corrected three-pointcompensation method. Two correction loops are shown. The first loop,shown in FIG. 51A, sets Tsub (substrate temperature) and Qoptical(optical illumination level) to mid-range. The bias coefficients arethen iterated such that the output signal is driven to mid-range. Theresult of this process is the centering of the dynamic range and thegeneration of "flat field" correction coefficients.

The second loop, shown in FIG. 51B, generates the bias-correctioncoefficients for the bias-corrected three-point method by collecting thedata necessary to choose the correction coefficients that provide thebest gain ratio temperature compensation. The "flat field" coefficients,minus the number of second loop iterations divided by two, are used asstarting bias-correction coefficient values. The second loop counter ispreset to the number of desired iterations (for example, 64 to 128,depending on the gain non-uniformity magnitude). At each iteration themicrobolometer array signal is measured for Qmin and Qmax for thesubstrate temperatures Tmin and Tmax. The bias-correction coefficientsare then incremented by 1 before the start of the next iteration. Afterall output data is acquired, the optical gain for each detector iscalculated for each bias-correction coefficient setting and eachsubstrate temperature. The mean optical gain is then calculated for Tminand Tmax using the "flat field" bias-correction coefficient values. Themean optical gain value at Tmin is then divided by the mean optical gainvalue at Tmax to establish the mean optical gain ratio. For everybias-correction coefficient setting used in the second loop, the ratioof optical gain at Tmin to optical gain at Tmax is calculated for eachdetector. The detector optical gain ratios are then analyzed to selectthe bias-correction coefficient value that provides the detector opticalgain ratio closest to the mean optical gain ratio between Tmin and Tmax.The resulting DAC coefficients are used to provide gain term correctionby applying a bias compensation to each detector.

IV. On-ROIC Substrate Temperature Control Method for MicrobolometerDetector Arrays

Due to non-uniformity correction limitations, previous microbolometerfocal plane arrays have required accurate control of the substratetemperature. By applying the pre-bias or bias-correction methods asdescribed, requirements for substrate temperature control can be relaxedsuch that on-ROIC substrate temperature control is now possible.

FIG. 52 illustrates a circuit to perform on-ROIC substrate temperaturecontrol. Here current source 40 supplies a constant current through athermally-shorted microbolometer 2. This circuit is used to sense thesubstrate temperature. An amplifier 42 drives an on-ROIC resistor 60 toheat the ROIC substrate to a constant temperature. A voltage source 43is used to set the desired substrate temperature. A low pass filter 41is used at the input of amplifier stabilize the thermal control loop.

FIG. 53 shows the resistor elements required to implement on the ROICcircuitry to control the temperature of the ROIC substrate 8. Shown arethe substrate temperature sensor resistor (thermally-shortedmicrobolometer 2) and the heating element resistor 60. This heatingelement will heat the substrate of the array and control the substratetemperature above room temperature. A material with a low thermalconductivity, such as silica glass 44, is used to provide a thermalstandoff from the surrounding environment.

The methods and circuitry disclosed in this application now make itfeasible to control the substrate of the array by heating instead ofcooling. The benefit of heating the substrate is that it is much easierand cheaper to control the substrate temperature by heating than bycooling, since the prior art required cooling below room temperature andcontrolling it to a fraction of a degree. The methods and circuitrydisclosed in this application therefore greatly simplify therequirements for the practical operation of microbolometer infraredfocal plane arrays.

V. On-ROIC Offset Correction Method for Microbolometer Detector Arrays

Many of the preceding embodiments show a voltage bias Vload connected tothe thermally-shorted microbolometer (see, e.g., FIGS. 21-24 and 29-35).The bias Vload is used to adjust the load current and thereby optimizethe operating point of the circuit by setting the output voltage orcurrent at a desired point within a range of outputs. In thoseembodiments, the bias Vload is typically a single voltage level set forthe entire array of microbolometers.

In further embodiments according to this invention a unique voltage biasis applied to each thermally-shorted microbolometer to provide a fineadjustment to the load voltage or current and thereby correct for offseterrors in the output signals from the thermally-isolatedmicrobolometers. This on-ROIC offset adjustment can supplant either theconventional offset correction to the output signal illustrated in FIGS.26 and 27 or both the conventional offset and gain corrections.

FIG. 54 illustrates a first embodiment of this aspect of the invention.It will be noted that FIG. 54 is identical to FIG. 29 except that asecond DAC 70 has been substituted for the bias Vload. DAC 70 provides aunique output voltage to the negative terminal of the thermally-shortedmicrobolometer 2.

As in FIG. 29, DAC 36 drives the gate of the common gate amplifier orMOSFET 31 so as to provide a variable bias for the thermally-isolatedmicrobolometer 3. The potential of the voltage source 11 and voltage atthe source of the P-channel MOSFET 31 establish the voltage across theactive microbolometer 3. The voltage at the drain of MOSFET 31 istypically biased such the MOSFET 31 is saturated. Since amplifier 32 hasa high-impedance input, the currents through microbolometers 2 and 3 areessentially the same. Thus the voltage across microbolometer 2 is equalto the product of (i) the current through microbolometer 3 and (ii) theresistance of microbolometer 2. The voltage at the input to amplifier 32is in turn equal to the sum of this product and the voltage provided byDAC 70. Since microbolometer 3 and the combination of microbolometer 3and MOSFET 31 act as a voltage divider, with the input to amplifier 32as the midpoint, the voltage at the input to amplifier 32 can beadjusted by changing the output of DAC 70. DAC 70 can be used in thisway in effect to remove the offset from the output of the circuit.

Another embodiment is shown in FIG. 55. The circuit of FIG. 55 issimilar to the circuit shown in FIG. 32, except that an N-channel MOSFET72 is connected between the drain of P-channel MOSFET 31 andmicrobolometer 2, the gate of MOSFET 72 being driven by a DAC 74 (also,row enable transistor 16 is not shown in FIG. 55).

While FIGS. 54 and 55 show the application of this aspect of theinvention on circuits of the kind shown in FIGS. 29 and 32,respectively, it will be understood that the adjustment of the offset byvarying the bias applied to the thermally-shorted microbolometer canalso be applied to other circuits, including those which provide anon-uniform bias to the thermally-isolated microbolometer (see, e.g.,FIGS. 30, 31, 34, 35), and those which provide a uniform bias to thethermally-isolated microbolometer (see, e.g., FIGS. 21-24).

Charge integrator or transimpedance amplifier 34A is connected to thecommon point between the drain of P-channel MOSFET 31 and N-channelMOSFET 72. Since the transimpedance amplifier 34A has a low-impedanceinput, the current flowing into transimpedance amplifier 34A is thedifference between the current in microbolometer 3 and the current inthermally-shorted microbolometer 2. The current through microbolometer 2is determined by the voltage across microbolometer 2, which isestablished by DAC 74 bias Vload. That voltage is equal to thedifference between -Vload and the voltage at the source of MOSFET 72which is essentially a threshold drop below the gate voltage applied byDAC 74. The current through microbolometer 3 is determined by theresistance of microbolometer 3 and the difference between the voltagesource 11 and the source potential of MOSFET 31 which is typicallyoperated in a saturated condition. The net effect is that the output ofDAC 74 can be used to control the portion of the current throughmicrobolometer 3 that flows into the inverting input of transimpedanceamplifier 34A.

The setting of DAC 70 in FIG. 54 is determined separately for eachmicrobolometer detector in the array and is applied synchronously withthe input to DAC 36. Similarly, the setting of DAC 74 in FIG. 55 isdetermined separately for each microbolometer detector in the array andis applied synchronously with the input to DAC 36.

These settings are determined by a calibration process which is designedto equalize the offsets of the outputs of the microbolometers in thearray. Initially, DAC 36 is calibrated as described above, with theinput to DAC 70 or 74 at mid-range. After DAC 36 has been calibrated,The substrate temperature is set to the midpoint between Tmin and Tmax,and the level of incident radiation is set to the midpoint between Qminand Qmax. DAC 70 or 74 is then incremented upward or downward by aniterative process until the output of amplifier 32 (in the embodiment ofFIG. 54) or the current into transimpedance amplifier 34A (in theembodiment of FIG. 55) is at the desired level.

The use of the "offset" DACs 70 and 74 may allow the conventional offsetcorrection illustrated in FIGS. 26 and 27 to be eliminated. Even if theconventional offset correction cannot be totally eliminated, the use ofDACs 70 and 74 greatly improves the uniformity and quality of thesignals at the outputs of the microbolometers. This in turn simplifiesthe circuitry that is required to perform the conventional corrections.

The offset and gain corrections are normally implemented as a part ofthe integration stage or after the integration stage, on or off of theROIC, and either in analog or digital form. The circuitry required forthese normal corrections is typically more expensive, more involved andmore complex than the additional circuitry required to perform offsetcorrection on the ROIC. This is particularly true if the system mustwithstand significant temperature differences.

Thus, the principles of this invention may be implemented in a widevariety of circuit devices and materials. Accordingly, the embodimentsdescribed above are only exemplary of the principles of the inventionand are not intended to limit the invention to the specific embodimentsdisclosed.

We claim:
 1. A microbolometer detector circuit comprising:a substrate; afirst microbolometer detector attached to but substantially thermallyisolated from said substrate; a second microbolometer detector thermallyshorted to said substrate, said second microbolometer detector being forproviding temperature compensation for said first microbolometerdetector; a first voltage source, said first and second microbolometersbeing connected in series in a conduction path supplied by said firstvoltage source; a transistor connected between said first and secondmicrobolometer detectors in said conduction path, a gate of saidtransistor being connected to a second voltage source; and a variablevoltage source coupled to said second microbolometer detector forproviding a variable voltage across said second microbolometer detector.2. The microbolometer detector circuit of claim 1 wherein said variablevoltage source comprises a digital-to-analog converter connected to anegative terminal of said second microbolometer detector.
 3. Themicrobolometer detector circuit of claim 1 wherein said variable voltagesource comprises a second transistor connected in series between saidfirst and second microbolometer detectors and a second digital-to-analogconverter connected to a gate of said second transistor.
 4. Themicrobolometer detector circuit of claim 1 wherein said second voltagesource comprises a digital-to-analog converter.
 5. The microbolometerdetector circuit of claim 4 wherein said digital-to-analog converter isfor providing a bias for correcting for variations in the properties ofsaid first microbolometer detector.
 6. The microbolometer detectorcircuit of claim 5 wherein said variable voltage source comprises asecond digital-to-analog converter connected to a negative terminal ofsaid second microbolometer detector.
 7. The microbolometer detectorcircuit of claim 5 wherein said variable voltage source comprises asecond transistor connected in series between said first and secondmicrobolometer detectors and a second digital-to-analog converterconnected to a gate of said second transistor.
 8. A microbolometerdetector circuit comprising:a substrate; a first microbolometer detectorattached to but substantially thermally isolated from said substrate; adigital-to-analog converter coupled to said first microbolometerdetector for providing a bias for correcting for variations in theproperties of said first microbolometer detector; and a secondmicrobolometer thermally-shorted to said substrate, said first andsecond microbolometer detectors being connected in series in aconduction path extending from an output of said digital-to-analogconverter; and a variable voltage source coupled to said secondmicrobolometer detector for providing a variable voltage across saidsecond microbolometer detector.
 9. The microbolometer detector circuitof claim 8 wherein said variable voltage source comprises a seconddigital-to-analog converter connected to a negative terminal of saidsecond microbolometer detector.
 10. The microbolometer detector circuitof claim 8 wherein said variable voltage source comprises a secondtransistor connected in series between said first and secondmicrobolometer detectors and a second digital-to-analog converter isconnected to a gate of said second transistor.
 11. A method of detectinga level of incident radiation comprising:providing a firstmicrobolometer detector, said first microbolometer detector beingattached to but substantially thermally isolated from a substrate;providing a second microbolometer detector, said second microbolometerdetector being thermally connected to said substrate; connecting saidfirst and second microbolometers in a series conduction path; applying afirst variable voltage to said first microbolometer detector; applying asecond variable voltage to said second microbolometer detector; anddetecting a resistance of said first microbolometer detector, saidresistance being representative of the level of said radiation.
 12. Themethod of claim 11 wherein the step of detecting the resistance of saidfirst microbolometer detector is performed during a sampling period, thesteps of applying said first and second variable voltages beingperformed during said sampling period.
 13. The method of claim 11wherein the step of detecting the resistance of said firstmicrobolometer detector is performed during a sampling period, the stepof applying said first variable voltage being performed before thebeginning of said sampling period, the step of applying said secondvariable voltage being performed during said sampling period.
 14. Themethod of claim 11 further comprising the step of performing a firstcalibration process to determine the level of said first variablevoltage, said first calibration process comprising the stepsof:detecting an output of said first microbolometer detector at a firstsubstrate temperature and at a first level of incident radiation;detecting an output of said first microbolometer detector at said firstsubstrate temperature and at a second level of incident radiation;detecting an output of said first microbolometer detector at a secondsubstrate temperature and at said first level of incident radiation;detecting an output of said first microbolometer detector at said secondsubstrate temperature and at said second level of incident radiation.15. The method of claim 14 further comprising the step of performing asecond calibration process to determine the level of said secondvariable voltage, said second calibration process comprising the stepsof:setting said first variable voltage at a level determined by saidfirst calibration process; detecting an output of said firstmicrobolometer detector at a first level of said second variablevoltage; and detecting an output of said first microbolometer detectorat a second level of said second variable voltage.
 16. The method ofclaim 15 wherein said microbolometer detector is included in an array ofmicrobolometer detectors and said first calibration process comprisesapplying a linear incremental technique to outputs of microbolometerdetectors in said array.
 17. The method of claim 15 wherein saidmicrobolometer detector is included in an array of microbolometerdetectors and said calibration process includes applying a recursivetechnique to outputs of microbolometer detectors in said array.